The present invention is an improvement over those dispersion imaging films, by way of example, disclosed in the aforesaid copending application Ser. No. 827,470, filed Aug. 25, 1977, now U.S. Pat. No. 4,211,838, granted July 8, 1980, and those disclosed in U.S. Pat. Nos. 4,082,861, granted Apr. 4, 1978, and 4,137,078, granted Jan. 30, 1979, as well as over those disclosed in said copending application Ser. No. 072,438. However, some aspects of this invention have applicability to other types of imaging films which utilize thin layers of material (such as metals, semiconductors or others), which are susceptible to degradation upon exposure to oxygen and/or water vapor in the atmosphere or otherwise. Thus, more generally, the present invention relates to improvements in imaging films carrying passivating layers for preventing or inhibiting the degradation of said imaging films with time due to moisture and/or oxygen which may gain access thereto.
The dispersion imaging films disclosed in the aforesaid application Ser. No. 827,470 and patents comprise a high optical density and substantially opaque layer of a dispersion imaging material deposited on a transparent or substantially transparent substrate and which, upon application of energy thereto in an amount sufficient to increase the absorbed energy in the opaque layer above a certain critical value, disperses or rolls-back to form a discontinuous layer comprising globules and free space therebetween which are frozen in place following the application of such energy and through which free space light can pass. (It should be understood that, in referring to a layer of imaging material, by "layer" is meant a body or film of imaging material which may be comprised of one homogeneous region of a given element or composition, or contiguous layered regions of different elements or compositions forming as a totality what may be considered or termed a single imaging layer of film.) Where a variation in the density of the image obtained is desired, there is produced dispersion inhibiting means for retarding the dispersion or roll-back thereof and for controlling the amount of such dispersion in accordance with the intensity of the applied energy above the certain critical value, to change the area of the openings in the opaque layer and, therefore, the average optical density of the various imaged portions thereof. Such an imaging film is referred to as a continuous tone film.
In high contrast imaging films, the parameters of the opaque layer of dispersion imaging material are such as to provide substantially no retarding of the roll-back of the material in its substantially fluid state from the initial openings therein, so that the roll-back is substantially instantaneous and substantially complete upon application of the applied energy above the certain critical value.
In both these high contrast and continuous tone imaging films, there is commonly provided a protective outermost layer of a suitable transparent synthetic plastic material which is generally permeable to air and moisture for protecting the opaque dispersion imaging film from abrasion damage. The substrate and outer protective layers of these imaging films most desirably are substantially colorless, transparent and flexible. The flexibility of the substrates and other layers of the films is necessary because, among other reasons, the films desirably are wound in rolls during manufacture, storage and shipment thereof. Also, flexibility of the thin outer protective layers of these films is necessary because they must conform without cracking to the variation in thickness of the opaque dispersion material at it disperses or balls-up in the imaging process. Colorless synthetic plastic materials are generally thermoplastic materials which have melting temperatures substantially less than 500.degree. C., which puts limitations on the imaging temperatures of the opaque dispersion film material deposited thereon. Thus, imaging temperatures must be sufficiently low that the substrate and outer protective layer will not be adversely affected by the imaging process.
The thin imaging layers of these dispersion and other types of imaging films are often unstable to long term exposure to air and/or water vapor. (These other types of films include certain light and heat processed films, and films which image by a change of morphological state, e.g. from a crystalline state to an amorphous structure.) Films such as these which are usually susceptible to oxidation and/or hydration or hydrolysis or other form of degradation require passivation layers on one or both sides with several specific requirements. The passivation layers must be continuous (i.e. have negligible holes or voids) and conform to the surface topology of the image layer to provide an effective barrier against the diffusion, for instance, of oxygen and/or water vapor. The passivation layers must also be flexible when the film is to be flexed when wound in a roll or where the changes in imaging layers geometry upon imaging require flexibility, especially for dispersion type films. Experience has shown that the required flexibility is achieved only in passivation layers having a thickness less than about 500 A.degree., and preferably 100 A.degree.-200 A.degree.. The passivation layers must have long term chemical stability, effective transparency, and must possess properties of adhesion to the adjacent imaging film layers consistent with the structural and photographic requirements of the film. Furthermore, it is desirable that the passivation layers act, in conjunction with the protective layer or layers, usually a polymer coating, to form an effective antireflective optical coating on the imaging layer, to allow the most efficient utilization of the incident energy. Finally, for cost-effective production, it is desirable to have layers which can be deposited rapidly and inexpensively, for example by vapor deposition using electron beam sources.
Amorphous dielectric films such as SiO, SiO.sub.2, TiO.sub.2, Si.sub.3 N.sub.4, Ta.sub.2 O.sub.5, etc. have been used for passivation in the semiconductor industry because of their chemical stability and the absence of grain boundaries through which vapor can diffuse. Similarly, more complex mixtures of oxides, such as pyrex glass, have been tried, but these applications used coatings many times thicker than the 100-200 A.degree. desirable for flexibility. Some of these passivating layers used in the semiconductor industry were layers of fused glass formed of various glass-forming oxides, like lead oxide, boron oxide, aluminum oxide, zinc oxide and silicon dioxide, reference being made, for example, to an article "Passivating Coatings on Silicon Devices" in the Journal of the Electrochemical Society, August, 1975. The application of fused glass layers using the conventional techniques described on page 1096 of this article result in film thicknesses of the order of magnitude of 10,000 A.degree.. While passivating layers made of these glassy materials formed in such thicknesses form good barriers to the passage of moisture and oxygen, they would be completely undesirable in the fabrication of dispersion imaging films of the kind described pursuant to the present invention. In the first place, as indicated above, the economical mass production and handling of imaging films generally requires that they be mountable in rolls which require that they have a high degree of flexibility. Also, passivating layers used in dispersion films must readily flex under the forces of the dispersion process. Fused glass layers of 10,000 A.degree. thick do not have this required flexibility. Moreover, when such passivating layers interface with the opaque dispersion imaging layers thereof, the effect thereof on the imaging characteristics becomes of importance. Such considerations are not present in the silicon devices with which these fused glass layers are utilized. Finally, to preserve the imaging characteristics of the imaging layers, the substrate temperature must be kept cool (below the imaging temperature) during deposition of the passivation layers. This requirement rules out conventional methods of depositing thicker fused glass coatings as well as chemical depositions which involve undesirably high substrate temperatures.
Said copending application Ser. No. 827,470 discloses the use of passivation layers composed of amorphous films of single oxides of semiconductors or metals (SiO, SiO.sub.2, Al.sub.2 O.sub.3, and GeO.sub.2). The use of these single component layers has the drawback that no single passivation material possesses all the desired passivation characteristics. When the passivating layers described extend along the faces of the opaque film of dispersion imaging material, they can have an effect upon the solid state interfacial adhesions between the substrate and the opaque layer deposited thereon and the protective layer deposited thereover. Generally speaking, poor solid state adhesion provides higher film sensitivity, while good solid state adhesion provides lower film sensitivity. Also, generally, SiO and SiO.sub.2 provide relatively poor solid state adhesion, while Al.sub.2 O.sub.3 and GeO.sub.2 provide relatively good solid state adhesion. GeO.sub.2 is flexible, continuous, and transparent, but tends to hydrolize and crystallize on long term exposure to water vapor.
Moreover, production costs of imaging films must be minimized. The most efficient way to produce imaging films is by a continuous mass production process in which the substrate material is unwound from a roll in a vacuum deposition chamber, where the various layers of material required on the substrate are deposited preferably by vapor deposition techniques (which are far more efficient than sputtering deposition techniques). It is thus desirable to use as thin a coating as possible of the passivating and other layers, and to increase the feeding speeds of the unwinding roll of substrate material past the deposition station involved. Thus, for example, it would be highly desirable to have passivating layers which have a thickness as little as 75-150 A.degree.. However, the possibility of providing continuous and stable passivating layers of such minute thicknesses which act as continuous barriers to the diffusion of moisture and oxygen would tend to be assumed, generally speaking, to be unlikely of attainment. In any event, especially in the case of the use of fused glass layers as passivating layers on a dispersion imaging film, because of the large thicknesses which were heretofore utilized for passivating layers in the completely different environment of silicon devices, such fused glass films as a passivating layer on dispersion imaging films would not be useful.
In said copending application Ser. No. 827,470, specific examples of passivating layer thicknesses given for the materials involved were of the order of magnitude of 150 A.degree.. While the passivating layers described therein are satisfactory under certain limited conditions, it was found that they had a less than desired shelf life for many applications. Of the various passivating layers described, the most preferred passivating material for interfacing with continuous tone opaque metal dispersion materials heretofore utilized was germanium oxide, because, as previously indicated, it provides an extremely flexible, thin, continuous layer (even for thicknesses as low as 75 A.degree.). Also, it has excellent adhesion to synthetic plastic material substrates and to the opaque metal dispersion materials found most useful in continuous tone imaging films, and thus either has no adverse effect upon and even sometimes improves the imaging quality of the opaque metal dispersion material. However, as indicated, it was found that the deposited germanium oxide layers tended to hydrolize and crystallize with time, and become cracked under the forces imparted thereto.
Germanium oxide is compatible with most continuous tone opaque dispersion materials because it does not adversely affect the desired controlled roll-back characteristics of such materials and such materials do not adversely interact with the germanium oxide. (A pure silicon dioxide passivating layer, on the other hand, because it offers little or no opposition to the roll-back of the opaque dispersion layer, was found unsatisfactory as a passivating layer interfacing with a continuous tone opaque dispersion layer.) Also, pure silicon dioxide has less than a desirable adhesion to metal surfaces and has less than the desired degree of flexibility. The other passivating layer materials described in said copending application Ser. No. 827,470, while operative and useful, were also found to be wanting in some important quality, like providing a continuous film in thicknesses much less than 200 A.degree., or because they readily recrystallize.
Accordingly, it is an object of the present invention to provide imaging films, such as dispersion imaging films, which include one or more passivating layers having a thickness no greater than about 500 A.degree., and preferably substantially less than 500 A.degree. like 200 A.degree. or less, and further wherein such passivating layers maintain their initial continuous, amorphous, barrier-forming character essentially indefinitely, or for prolonged periods of time so that the imaging film has a very long shelf life.
Another object of the invention is to provide imaging films as described where the passivating layer interfaces the dispersion imaging layer thereof, and is not adversely affected thereby or adversely affects the desired imaging qualities thereof.